The former first secretary of state, and one of the Prime Minister’s closest friends and allies, had faced a very public probe over claims explicit material was found in 2008.

And he quit the Cabinet last night at the Prime Minister's request, apologising for his conduct, after independent adviser on ministers' interests, Sir Alex Allan, found he had breached the ministerial code and misled the public.

Mr Green, 61, MP for Ashford since 1997, is the third minister in Mrs May’s cabinet to resign in just two months, following the exits of Sir Michael Fallon and Priti Patel. 

And his fall from office is seen by many as the culmination of a decade-long feud with former Met Police chief Bob Quick, the officer who revealed that 'extreme' porn was found on Mr Green's Commons computer during a raid on his parliamentary office back in 2008.  

Today a friend of Mr Green reportedly said the police had ‘got their man after a nine-year vendetta’.  

The probe found statements given by Mr Green saying he was not aware that indecent material found on his parliamentary computers “were inaccurate and misleading”.

The investigation found the Met Police had previously informed him the material existed.

But it could not reach a definitive conclusion on alleged advances towards journalist Kate Maltby in 2015.

Sir Alex's report said there had been "competing and contradictory accounts of what were private meetings" between the pair but said the journalist’s claims had been “plausible".

Mr Green has said he regrets the "distress caused" to Ms Maltby but did not recognise her version of events.

The Prime Minister, who asked him to stand down, said she was "extremely sad" at his exit.

Mr Green has described allegations that he downloaded or viewed pornography on his parliamentary computer as "unfounded and deeply hurtful", but admitted he had not been clear in his past statements on the issue.

In his resignation letter, Mr Green wrote: “I regret that I've been asked to resign from the government following breaches of the Ministerial Code, for which I apologise.”

He went on: “From the outset I have been clear that I did not download or view pornography on my Parliamentary computers. 

“I accept that I should have been clear in my press statements that police lawyers talked to my lawyers in 2008 about the pornography on the computers, and that the police raised it with me in a subsequent phone call in 2013. 

“I apologise that my statements were misleading on this point. 

“The unfounded and deeply hurtful allegations that were being levelled at me were distressing both to me and my family and it is right that these are being investigated by the Metropolitan Police's professional standards department.”

In her own letter to Mr Green, Mrs May said she regretted losing Mr Green, a close ally since their university days together.

She told him: “I have also carefully considered the report's conclusions in relation to two statements you made on 4 and 11 November which you now accept were inaccurate and misleading. 

“This falls short of the Seven Principles of Public Life and is a breach of the Ministerial Code - a conclusion which has been endorsed by Sir Alex Allan, the Independent Adviser on Ministers' Interests. 

“While I can understand the considerable distress caused to you by some of the allegations which have been made in recent weeks, I know that you share my commitment to maintaining the high standards which the public demands of Ministers of the Crown.

“It is therefore with deep regret, and enduring gratitude for the contribution you have made over many years, that I asked you to resign from the Government and have accepted your resignation.”

The inquiry was triggered after Kate Maltby, who is three decades younger than the 61-year-old, claimed Mr Green "fleetingly" touched her knee during a meeting in a pub.

And she said a year later he sent her a "suggestive" text message after she was pictured wearing a corset in a newspaper.

Mr Green has always denied the allegations.

A source close to Brexit Secretary David Davis confirmed he would not quit the Government in protest at Mr Green's sacking, despite a report earlier this month that he had threatened to resign if that happened.

Mr Davis was Mr Green's boss as shadow home secretary at the time of the raid.

The resignation of one of her most trusted allies, who helped pacify her deeply divided party, is a serious blow for Mrs May as she navigates the final year of tortuous Brexit negotiations before Britain's exit in March 2019.

Mr Green was appointed in the wake of her disastrous decision to call a June snap election which lost her party its majority in Parliament. 

Her old ally was brought in to stabilise her leadership and counter enemies within the Conservative Party who wanted her to quit.

Mr Green is the second Cabinet Minister to resign over the Westminster sexual harassment scandal, after Sir Michael Fallon quit as Defence Secretary on November 1.

He said his behaviour had "fallen below the high standards required" after he admitted putting his hand on the knee of radio presenter Julia Hartley-Brewer some years ago.

He was among a number of politicians caught in a wave of allegations of improper behaviour.